(a) Field of the Invention
The present invention relates to a liquid crystal display apparatus and a method of driving the same, and more particularly to a method of driving a liquid crystal display element which provides high contrast and high brightness and which is not affected by electrical asymmetry, as well as a liquid crystal display apparatus having a liquid crystal display element that is driven by such a method.
(b) Description of the Related Art
The mainstream of a high performance liquid crystal display apparatus is a TFT (thin-film transistor)-scheme active matrix liquid crystal display apparatus of a TN (twisted nematic) mode using nematic liquid crystal or an IPS (in-plane switching) mode. In such an active matrix liquid crystal display apparatus, an image is re-displayed at 60 Hz, because positive and negative image signals are written at 30 Hz, making the time period of one field about 16.7 ms (millisecond). The total of the time for writing the positive image signal and the time for writing the negative image signal is called a frame time, which is about 33.3 ms. By contrast, the response time of the fastest available liquid crystal is almost equal to the frame time. Therefore, display of an image including a motion picture or display of a high speed computer image requires a response speed faster than the current frame time.
Meanwhile, a field-sequential color liquid crystal display apparatus has been studied in an effort to increase resolution. In the field-sequential color liquid crystal display apparatus, the color of a back light of the liquid crystal display apparatus is sequentially switched among red, green, and blue. Since this method does not require that color filters be spatially disposed, the resolution can be increased to three times that of a conventional liquid crystal display apparatus. In the field-sequential color liquid crystal display apparatus, since an image for one color must be displayed within a period ⅓ that for one field, the time period that can be used for display is about 5 ms. Therefore, liquid crystal itself is required to have a response time shorter than 5 ms. Liquid crystal that causes spontaneous polarization, such as ferroelectric liquid crystal or antiferroelectric liquid crystal, has been studied as candidate liquid crystal capable of achieving such a high response speed. Further, in relation to nematic liquid crystal, various studies have been performed in an effort to improve a response speed through an increase in the degree of dielectric anisotropy, a decrease in viscosity and/or thickness, or employment of a pi-type liquid crystal orientation.
In an active matrix liquid crystal display element, the operation of storing a voltage and a charge in the liquid crystal section is actually performed only in a period during which each scan line is selected (write time). The write time is 16.7 μs (microsecond) in the case of a liquid crystal display apparatus which has 1000 lines and in which an image signal for the 1000 lines is written within one field time, and about 5 μs in the case of a liquid crystal display apparatus which is driven in a field sequential scheme. Presently, no known liquid crystal element or known manner of liquid crystal completes its response within the time period as described above. Even among liquid crystal elements that cause spontaneous polarization and nematic liquid crystal of improved response speed, no known element exhibits such quick response. This results in the problem that response of liquid crystal generally occurs after completion of signal write operation. Consequently, in liquid crystal elements that cause spontaneous polarization, a depolarization field is generated due to rotation of spontaneous polarization, so that voltages at opposite ends of a liquid crystal layer drop abruptly. Therefore, the voltages stored at the opposite ends of the liquid crystal layer change largely. Meanwhile, in the high speed nematic liquid crystal, change in the capacitance of a liquid crystal layer caused by anisotropy of dielectric constant increases considerably, resulting in a change in the voltage that is written into the liquid crystal layer and must be held constant. Such a decrease in the holding voltage; i.e., a decrease in the effective applied voltage, results in insufficient writing, so that the on-screen contrast decreases. Further, when the same signal is repeatedly written, the brightness continuously changes until lowering of the holding voltage stops, so that a few frames are required to obtain stable brightness.
“Japanese Applied Physics,” Vol. 36, Part 1, No. 2, pp 720-729 reports that a so-called “step response” phenomenon occurs when an identical image signal is written over a few frames after a frame in which an image signal changes and thus the absolute value of a signal voltage changes. According to this phenomenon, for the same signal voltage, the transmittance of liquid crystal changes in the manner of damped oscillation over a few frames, so that the liquid crystal becomes bright in alternate frames and dark in other frames. After a few frames, the transmittance is stabilized at a predetermined level.
An example of the above phenomenon will be described with reference to FIGS. 1-3. FIG. 1(a) is chart showing the waveform of a data voltage; and FIG. 1(b) is a chart showing change in transmittance at that time. When the data voltage shown in FIG. 1(a) is applied to liquid crystal, as shown in FIG. 1(b), the transmittance of liquid crystal changes in the manner of damped oscillation such that the liquid crystal becomes alternately light and dark. In the illustrated example, the transmittance of the liquid crystal converges to a constant level in the fourth frame. Since the liquid crystal requires a few frames to change its transmittance as described above, high speed display of images is impossible.
FIG. 2(a) is a chart showing the waveform of a data voltage; FIG. 2(b) is a chart showing change in a gate voltage; and FIG. 2(c) is a chart showing change in transmittance at that time. FIG. 3 is a timing chart for scan lines in the drive shown in FIG. 2. The color shade during each of positive and negative display periods 102 and 104 represents brightness corresponding to the transmittance of FIG. 2. In FIG. 3, a time period of 16.7 ms is indicated by an arrow.
FIG. 3 depicts six scan lines. Positive writing 101 is successively performed from the top scan line in order to obtain a positive display 102, and then negative writing 103 is successively performed from the top scan line in order to obtain a negative display 104. In each scan line, the period of the positive writing 101 and the period of the positive display 102 constitute a first field, while the period of the negative writing 103 and the period of the negative display 104 constitute a second field. The first and second fields constitute one frame.
When the data voltage of FIG. 2(a) is applied and a TFT switch is turned on by the gate voltage of FIG. 2(b), as shown in FIG. 2(c), the transmittance of liquid crystal changes in the manner of damped oscillation such that the liquid crystal becomes alternately light and dark. This is observed as flicker, which has the effect of deteriorating the quality of display. Further, as shown in FIG. 2(c), the transmittance of the liquid crystal converges to a constant level in the second frame (fourth field) following application of the signal voltage.
As a result, the brightness changes in an oscillating manner, as shown in FIG. 3. As described above, even when liquid crystal of high response speed is used, the speed of a display image decreases, because a few frames are required to stabilize the brightness.
The transmittance of liquid crystal after response is determined not by an applied signal voltage but by the amount of charge stored in the liquid crystal serving as a capacitor. The amount of charge depends on a total amount of accumulated charge that exists before the signal is written and of charge that is newly written. The amount of charge accumulated after response also changes depending on design values in relation to pixels such as the physical constants of liquid crystal, electric parameters and an amount of charge accumulation. Therefore, in order to establish correspondence between a signal voltage and transmittance, data, actual calculation, and the like are required for determining (1) the relationship between the signal voltage and an amount of stored charge, (2) an amount of charge present before signal writing operation, and (3) an amount of charge present after response. Therefore, there becomes necessary a frame memory for storing the data regarding (2) for the entire screen, and a calculation section for calculating the data (1) and (3). This is not preferred, because the number of parts of the system increases.
In order to solve the above problem, there is sometimes used a reset pulse scheme, in which a reset voltage is applied to liquid crystal so as to bring the liquid crystal into a predetermined state before new data are written therein.
As an example, a technique described in IDRC, pp. 66-69, 1997 will be described. This technique uses an OCB (optically compensated bi-refligence) mode in which nematic liquid crystal is aligned to obtain a pi-type alignment, and compensation film is attached to the liquid crystal. The response speed of the liquid crystal mode is about 2 to 5 msec, which is considerably faster than that of a conventional TN mode.
Although response is theoretically considered to be completed within one frame, as described above, a few frames are required for attainment of stable transmittance, because a holding voltage greatly decreases due to change in dielectric constant caused by response of the liquid crystal. A method for solving this problem is shown in FIG. 5 of the above literature. In this method, within one frame, a signal for black display is always written after a signal for white display is written. FIG. 5 of the literature is reproduced as FIG. 4. The horizontal axis represents time, while the vertical axis represents brightness. A dotted line shows variation in brightness for the case of ordinary drive and indicates that the brightness reaches a stable level in the third frame.
When the reset pulse scheme is employed, liquid crystal always attains a predetermined state before new data are written therein, and one-to-one correspondence can be observed between a written signal voltage and an obtained transmittance. This one-to-one correspondence simplifies the manner of generation of drive signals and obviates a frame memory or other means for storing previous written information.
In order to apply a reset voltage to liquid crystal, there is used another method which comprises the steps of generating positive and negative data signal voltages for a certain image signal; applying to the liquid crystal the positive (negative) voltage and then the negative (positive) voltage; and subsequently applying a reset voltage to the liquid crystal. In this case, if the positive and negative data signal voltages having the same amplitude are applied, the “step response” as described above occurs. Therefore, a data signal voltage having a waveform shown in FIG. 5(a) is applied to liquid crystal. FIG. 5(b) is a graph showing a variation in transmittance observed at that time. The waveform of a data signal voltage whose negative and positive values have the same amplitude is indicated by a dotted line in FIG. 5(a), and a variation in transmittance when the data signal voltage of FIG. 5(a) is applied to liquid crystal is indicated by a dotted line in FIG. 5(b).
In order to avoid “step response,” as shown in FIG. 5(a), the amplitude of a data voltage in a first half of a frame (a positive data voltage in this example) is made small, and the amplitude of the data voltage in a second half of the frame (a negative data voltage in this example) is made equal to that of the waveform indicated by the dotted line. This setting prevents step response, so that, as shown in FIG. 5(b), the same transmittance is obtained in the first and second halves-of the frame. By the subsequent step of resetting the liquid crystal at the end of the frame, the liquid crystal is brought into a predetermined reset state. In a subsequent frame, a new signal voltage having a similar waveform is applied, so that the transmittance of the liquid crystal changes in accordance with the new signal voltage. In this manner, one-to-one correspondence is established between the constant signal voltage and the constant transmittance.
Further, in order to solve these problems, there has been proposed a drive method called “pseudo DC-drive” shown in AMLDC, 97 digest, pp. 119-122.
This technique will be described with reference to FIGS. 6 and 7. Similar to FIG. 2, FIG. 6(a) is a chart showing the waveform of a data voltage; FIG. 6(b) is a chart showing change in a gate voltage; and FIG. 6(c) is a chart showing change in transmittance at that time. FIG. 7 is a timing chart for each scan line, and the color shade in each of positive and negative display periods 102 and 104 represents brightness corresponding to the transmittance of FIG. 6(c). In FIG. 6, a time period of 16.7 ms is indicated by an arrow.
In the literature, a period of 16.7 ms is defined as one frame period. However, since this definition is not generally accepted, the period is changed in the drawings of the present specification (one frame period described in the literature corresponds to one field period used in the present specification with regard to ordinary conventional techniques).
In the “pseudo DC-drive” unlike the case of AC drive as shown in FIG. 2, a data voltage of the same polarity is continuously applied to liquid crystal over a plurality of fields. After the plurality of fields, the polarity of the data voltage is reversed so as to eliminate electrical imbalance. In FIG. 6, after positive writing over four fields, negative writing is performed over four fields to complete display of one image signal. Since writing is performed for each scan line at timing as shown in FIG. 7, an operation of successively writing positive data from the top line is repeated four times, and then an operation of successively writing negative data from the top line is repeated four times.
This method enables attainment of a state in which voltages held at opposite ends of liquid crystal become the same as a constant applied DC voltage. As a result, the holding voltage does not decrease due to response of liquid crystal, and the final transmittance becomes higher than that in the case of AC drive shown in FIG. 2, in which the holding voltage decreases due to response of liquid crystal.
However, in this method, one frame period becomes equal to the total of a plurality of frames of different polarities. That is, in the example shown in FIG. 6, the length of one frame is four times that of the frame in FIG. 2.
Even if any of the reset pulse schemes described above is employed, the conventional reset pulse method has the following problems. First, brightness changes greatly depending on position within a screen which is effected by the timing when the reset operation is performed. For example, when scanning is performed from the top of the screen toward the bottom of the screen, and the reset operation is performed after completion of scanning of all lines; at the top of the screen, a period substantially corresponding to one field is available as a display time after the writing operation, but at the bottom of the screen, only a very short time is available as the display time after the writing operation. This phenomenon is described with reference to FIG. 8.
FIG. 8(a) schematically shows states in a write (scanning) period 101, a display period 102, and a reset period 103 in two dimensions; i.e., the scanning direction of a screen and time axis. In this Figure, eight scan lines are shown, and in the write period 101, scanning is performed successively from the top of the screen toward the bottom thereof. After the display period 102 having a predetermined length, the entire screen is reset at a time during the reset period 103. FIG. 8(b) schematically shows a scan line voltage and transmittance in the uppermost part of the display or on a first (No. 1) scan line when white color is displayed by use of the drive method as described above. FIG. 8(c) schematically shows a scan line voltage and transmittance in the lowermost part of the display or on an eighth (No.8) scan line. In the first scan line, white is displayed during a relatively long period corresponding to a value equal to one frame period less the sum of the reset period and a transient response period. However, in the eighth scan line, since the reset is started simultaneously with the end of the response period, white cannot be displayed sufficiently. As a result, when the entire frame period is considered, as shown in FIG. 9B, there occurs a phenomenon that the top portion of the screen is bright and the bottom portion of the screen is dark. Such a brightness variation within the screen deteriorates image quality considerably.
Next, since the period for bringing the liquid crystal into a predetermined display state always exists, the overall contrast and the maximum transmittance decrease. For example, if the liquid crystal is reset such that the liquid crystal display turns to black, a period available for displaying a certain color other than black becomes shorter than that available when no reset operation is performed, so that the maximum transmittance and the transmittance at each gradation both decrease. If the liquid crystal is reset such that the liquid crystal displays a color other than black, the transmittance at the time of the reset is added when black is displayed and is averaged with respect to time, with the result that the transmittance at the time of black being displayed is increased, and the contrast decreases.
Further, since the period during which the transmittance of the liquid crystal attains a constant level always exists, flicker is generated between that transmittance and a transmittance occurring at the time of another color being displayed. For example, when the entire screen is reset concurrently, flickering occurs over the entire screen, so that a great degree of flicker is observed.
Moreover, the scanning period decreases by an amount corresponding to the length of the reset period. In general, the scanning period (write time) is substantially equivalent to a time obtained through division of the field time, which is half the frame time, by the number of scan lines. However, if a reset period is provided in the field time, the scanning period 101 shown in FIG. 8(a) decreases to a time obtained through division of (the field time minus the reset time 103) by the number (8) of scan lines. As a result, the scanning period becomes shorter. In order to solve the problem that the reset period affects the scanning period, there has been proposed a method in which interlace drive is combined with reset, as shown in, for example, Patent Publication JP-A-92-186217. In this method, a FLC (ferroelectric liquid crystal) panel is driven in an interlace mode, and scan lines are reset in their respective non-display periods. This prevents a decrease in the length of the scanning period. Further, the reset timings of adjacent lines are shifted from each other, and the degree of flicker is considered to decrease due to averaging. However, even when this method is used, the remaining problems, such as variation of brightness within the screen and decrease in the maximum transmittance, cannot be solved.
Meanwhile, in the pseudo DC-drive, as described above, a longer frame period (in the example of FIGS. 6 and 7, a period four times that of an AC drive) is required compared to the AC drive, so that high responsiveness of the liquid crystal cannot be exploited effectively. Consequently, flicker of a long period, which fluctuates at a period a few times that of the ordinary frame period (16.7 ms), is generated of which brightness is shown in FIG. 7.